1. Field of the Invention
The present invention relates to an optical transmission apparatus provided with a plurality of modulating sections, and in particular, to an optical transmission apparatus capable of compensating for the delay deviation depending on the temperature variation between drive signals.
2. Description of the Related Art
In recent years, there have been increased demands for introducing a 40 Gbit/s optical transmission system of next generation, and furthermore, such a system is required to achieve a transmission distance and frequency utilization efficiency equivalent to those of a 10 Gbit/s system. As means for realizing such a system, there has been actively performed the search and development of the RZ-DPSK (Differential Phase Shift Keying) modulation or the CSRZ-DPSK modulation, which is a modulation format with excellent optical signal-to-noise ratio (OSNR) efficiency and excellent non-linear tolerance, in comparison with a NRZ (Non Return to Zero) modulation format which has been applied to a conventional system of 10 Gbit/s or lower (refer to the literature 1: T. Hoshida et al., “Optimal 40 Gb/s Modulation Formats for Spectrally Efficient Long-Haul DWDM Systems”, Journal of Lightwave Technology, vol. 20, No. 12, pp. 1989-1996, December 2002, and the literature 2: O. Vassilieva et al., “Non-Linear Tolerant and Spectrally Efficient 86 Gbit/s RZ-DQPSK Format for a System Upgrade”, OFC 2003, ThE7, 2003). Moreover, in addition to the above modulation format, there has also been actively performed the search and development of a phase modulation format such as the RZ-DQPSK (Differential Quadrature Phase-Shift Keying) modulation or a CSRZ-DQPSK modulation, which has a feature of narrow spectrum (high frequency utilization efficiency).
FIG. 22 is a diagram showing a configuration example of an optical transmission apparatus and an optical reception apparatus to which a RZ-DPSK or CSRZ-DPSK modulation format of 43 Gbit/s is adopted. Further, FIG. 23 is a graph showing states of optical intensity and optical phase in the case where a RZ-DPSK or CSRZ-DPSK modulated optical signal is transmitted/received.
In FIG. 22, an optical transmission apparatus 110 is for transmitting an optical signal in the RZ-DPSK or CSRZ-DPSK modulation format of 43 Gbit/s, and comprises, for example, a transmission data processing section 111, a CW (Continuous Wave) light source 112, a phase modulator 113 and a RZ pulsing intensity modulator 114.
To be specific, the transmission data processing section 111 is provided with a function as a framer for framing input data and a function as a FEC (Forward Error Correction) encoder for giving an error-correcting code, and also, is provided with a function as a DPSK pre-coder for performing the coding processing reflected with information of a difference between a code of 1 bit before and a current code.
The phase modulator 113 phase modulates a continuous light from the CW light source 112 in accordance with coded data from the transmission data processing section 111, to output an optical signal in which optical intensity thereof is fixed but a binary optical phase thereof carries information, that is, a DPSK modulated optical signal (refer to a lower stage of FIG. 23).
The RZ pulsing intensity modulator 114 is for RZ pulsing the optical signal from the phase modulator 113 (refer to an upper stage of FIG. 23). In particular, an optical signal which is RZ pulsed using a clock drive signal having a frequency (43 GHz) same as a bit rate of data and also having the amplitude of 1 time an extinction voltage (Vπ), is called a RZ-DPSK signal, and further, an optical which is RZ pulsed using a clock drive signal having a frequency (21.5 GHz) half the bit rate of data and also having the amplitude of 2 times the extinction voltage (Vπ), is called a CSRZ-DPSK signal.
Further, an optical reception apparatus 130 is connected to the optical transmission apparatus 110 via a transmission path 120 and an optical repeater 121, to perform the reception processing on the (CS)RZ-DPSK signal from the optical transmission apparatus 110, which has been optical repeatedly transmitted, and comprises, for example, a delay interferometer 131, a photoelectric converting section 132, a regeneration circuit 133 and a received data processing section 134.
To be specific, the delay interferometer 131 is configured by, for example, a Mach-Zehnder interferometer, and for the (CS)RZ-DPSK signal transmitted via the transmission path 120, makes a delay component per 1 bit time (23.3 ps in the configuration example of FIG. 22) and a component phase controlled of 0 rad to interfere (delay interfere) with each other, to output interference results thereof as two outputs. Incidentally, the above Mach-Zehnder interferometer is configured so that one of branched waveguides is formed to be longer than the other branched waveguide by the propagation length equivalent to the 1 bit time, and also, is formed with an electrode for phase controlling the optical signal propagated through the other branched waveguide.
The photoelectric converting section 132 is configured by a dual pin photodiode for receiving the respective outputs from the delay interferometer 131 to perform the differential photoelectric conversion detection (balanced detection). Incidentally, received signals detected by the photoelectric converting section 132 are appropriately amplified by an amplifier.
The regeneration circuit 133 is for extracting a data signal and a clock signal from the received signals which are subjected to the differential photoelectric conversion detection in the photoelectric converting section 132.
The received data processing section 134 is for performing the signal processing, such as an error correction and the like, based on the data signal and the clock signal extracted by the regeneration circuit.
FIG. 24 is a diagram showing a configuration example of an optical transmission apparatus and an optical reception apparatus to which a RZ-DQPSK or CSRZ-DQPSK modulation format of 43 Gbit/s is adopted. Further, FIG. 25 is a graph showing states of optical intensity and optical phase in the case where a RZ-DQPSK or CSRZ-DQPSK modulated optical signal is transmitted/received. Incidentally, the configuration of the optical transmission and reception apparatuses corresponding to the RZ-DQPSK or CSRZ-DQPSK modulation format is recited in detail in Japanese National Phase Patent Publication No. 2004-516743, and therefore, the outline thereof will be described here.
In FIG. 24, an optical transmission apparatus 210 comprises, for example, a transmission data processing section 211, a 1:2 demultiplexing section (DEMUX) 212, a CW light source 213, a π/2 phase shifter 214, two phase modulators 215A and 215B, and a RZ pulsing intensity modulator 216.
To be specific, similarly to the transmission data processing section 111 shown in FIG. 22, the transmission data processing section 211 is provided with functions as a framer and a EFC encoder, and also, is provided with a function of a DQPSK pre-coder for performing the coding processing reflected with information of a difference between a code of 1 bit before and a current code.
The 1:2 demultiplexing section 212 is for demultiplexing coded data of 43 Gbit/s from the transmission data processing section 211 into coded data #1 and #2 in dual series of 21.5 Gbit/s.
The CW light source 213 is for outputting a continuous light, and the output continuous light is separated into two, so that one of the separated lights is input to the phase modulator 215A, and the other is input to the phase modulator 215B via the π/2 phase shifter 214.
The phase modulator 215A modulates the continuous light from the CW light source 213 using the coded data #1 being one of the dual series, which is demultiplexed by the 1:2 demultiplexing section 212, to output an optical signal in which a binary optical phase (0 rad or π rad) thereof carries information. Further, the phase modulator 215B is input with a light which is obtained by phase shifting by π/2 the continuous light from the CW light source 213 in the π/2 phase shifter 214, and modulates this input light using the coded data #2 being the other series, which is demultiplexed by the 1:2 demultiplexing section 212, to output an optical signal in which a binary optical phase (π/2 rad or 3π/2 rad) thereof carries information. The lights modulated by the phase modulators 215A and 215B are multiplexed with each other, to be output to the latter stage RZ pulsing intensity modulator 216. Namely, the modulated lights from the phase modulators 215A and 215B are multiplexed with each other, so that an optical signal in which optical intensity thereof is fixed but a four-valued optical phase thereof carries information (refer to a lower stage of FIG. 25), that is, a DQPSK modulated optical signal, is sent to the RZ pulsing intensity modulator 216.
The RZ pulsing intensity modulator 216, similarly to the RZ pulsing intensity modulator 114 shown in FIG. 22, is for RZ pulsing the DQPSK modulated signal from the phase modulators 215A and 215B. In particular, an optical signal which is RZ pulsed using a clock drive signal having a frequency (21.5 GHz) same as a bit rate of data #1 and data #2, and also having the amplitude of 1 time an extinction voltage (Vπ), is called a RZ-DQPSK signal, and further, an optical which is RZ pulsed using a clock drive signal having a frequency (10.75 GHz) half the bit rate of data #1 and data #2, and also having the amplitude of 2 times the extinction voltage (Vπ), is called a CSRZ-DQPSK signal.
Further, an optical reception apparatus 230 is connected to the optical transmission apparatus 210 via a transmission path 220 and an optical repeater 221, to perform the reception processing on the (CS)RZ-DQPSK signal from the optical transmission apparatus 210, which has been optical repeatedly transmitted, and comprises, for example, a branching section 231 that branches the received optical signal into two, and also comprises, on optical signal paths through which the branched optical signals are respectively propagated, delay interferometers 232A and 232B, photoelectric converting sections 233A and 233B, and regeneration circuits 234A and 234B, and further, comprises a 2:1 multiplexing section (MUX) 235 that multiplexes the data signals regenerated in the regeneration circuits 234A and 234B, and a received data processing section 236.
To be specific, the delay interferometers 232A and 232B receive respectively the optical signals which are obtained by branching the (CS)RZ-DQPSK signal transmitted through the transmission path 220 and the optical repeater 221 into two by the branching section 231. The delay interferometer 232A makes a delay component per 1 bit time (46.5 ps in the configuration example of FIG. 24) and a component phase controlled of π/4 rad to interfere (delay interfere) with each other, to output interference results thereof as two outputs. Further, the delay interferometer 232B makes a delay component per 1 bit time and a component phase controlled of −π/4 rad (which is shifted by π/2 rad to the phase controlled component in the delay interferometer 232A) to interfere (delay interfere) with each other, to output interference results thereof as two outputs. Here, the delay interferometers 232A and 232B each is configured by a Mach-Zehnder interferometer, and each Mach-Zehnder interferometer is configured so that one of branched waveguides is formed to be longer than the other branched waveguide by the propagation length equivalent to the 1 bit time, and also, is formed with an electrode for phase controlling the optical signal propagated through the other branched waveguide.
The photoelectric converting sections 232A and 232B are respectively configured by dual pin photodiodes for receiving the respective outputs from the delay interferometers 232A and 232B to perform the differential photoelectric conversion detection. Incidentally, received signals detected respectively by the photoelectric converting sections 233A and 233B are appropriately amplified by amplifiers.
The regeneration circuit 234A is for regenerating an in-phase component I for a clock signal and a data signal from the received signal detected by the differential photoelectric conversion in the photoelectric converting section 233A. Further, the regeneration circuit 234B is for regenerating a quadrature-phase component Q for a clock signal and a data signal from the received signal detected by the differential photoelectric conversion in the photoelectric converting section 233B.
The 2:1 multiplexing section 235 is for receiving the in-phase component I and the quadrature-phase component Q from the regeneration circuits 234A and 234B, to convert these components into a data signal of 43 Gbit/s before DQPSK modulation.
The received data processing section 234 is for performing the signal processing, such as an error correction and the like, based on the data signal from the 2:1 multiplexing section 235.
The optical transmission apparatuses of (CS)RZ-DPSK modulation format and (CS)RZ-DQPSK modulation format as described in the above, each has a configuration in which a plurality of optical modulators are serially arranged. In the modulation format using such a plurality of optical modulators, there is a problematically possibility that the variation in optical signal delay amount generated among the plurality of optical modulators causes the signal deterioration. Such a problem is common to the cases where the transmission of high bit rate such as 40 Gbit/s or the like is performed in the modulation format using a plurality of optical modulators. Other than the above described (CS)RZ-DPSK modulation format and the (CS)RZ-DQPSK modulation format, there is also a possibility that the variation in optical signal delay amount among the plurality of optical modulators causes the signal deterioration, in a RZ (Return-to-zero) modulation format (refer to the literature: A. Sano et al., “Performance Evaluation of Prechirped RZ and CS-RZ Formats in High-Speed Transmission Systems with Dispersion Management”, Journal of Lightwave Technology, Vol. 19, No. 12, pp. 1864-1871, December 2001), a CS-RZ (Carrier-suppressed Return-to-zero) modulation format (refer to the literature: Y. Miyamoto et al., “1.2 Tbit/s (30×42.7 Gbit/s ETDM optical channel) WDM transmission over 376 km with 125 km spacing using forward error correction and carrier-suppressed RZ format”, Optical Fiber Communication Conference 2000 (OFC) 2000, Pd26, 2000, an OTDM (Optical time domain multiplexing) modulation format (refer to the literature: G. Ishikawa et al., “80-Gb/s (2×40-Gbs) Transmission experiments over 667-km Dispersion-shifted fiber using Ti:LiNbO3 OTDM modulation and demultiplexer”, ECOC '96 ThC3.3, 1996) or the like.
As a conventional technology coping with the above problem, as shown in FIG. 26 for example, there has been proposed a configuration in which phases of clock signals each applied on a phase modulator 312 and an intensity modulator 313 which are sequentially connected between a CW light source 311 and an output terminal, are compared with each other by a mixer 314, and a shift amount of a phase shifter 316 is controlled by an automatic delay compensation circuit (ADC) 315 based on a phase comparison result, so that a phase relation between both of the clock signals is held at a fixed value (refer to Japanese Unexamined Patent Publication No. 2002-353896).
However, the conventional technology as described above is a system for directly monitoring the drive signals applied on the plurality of modulators to detect a relative phase relation (differential delay), to thereby perform a feedback control based on a detection result. Therefore, there is a problem in that although the delay deviation at an electric level can be compensated, the delay deviation at an optical level cannot be compensated. For the delay deviation at the optical level, as shown in FIG. 27 for example, there is a problem in that an optical propagation delay amount in a polarization maintaining fiber (PMF) 414 connecting optical modulators 412 and 413 to each other is changed depending on the temperature. FIG. 28 shows one example of a measurement result of a delay amount relative to a temperature change (reference temperature: 25° C.), for a PMF using polyester elastomer as fiber coating. Note, a wavelength of a light is 1550 nm. From the measurement result in FIG. 28, it is understood that the delay amount is increased depending on the temperature rise.
FIG. 29 is a graph rounding up measurement results of temperature dependence of delays caused in various types of PMF and a LN modulator provided with a PMF. From FIG. 29, it is understood that delay variation amounts in PMFs are changed depending on types of PMF coating. To be specific, the delay variation amount of the PMF of 400 μm coating diameter using an ultraviolet curing resin (UV) is smaller that of the PMF of 900 μm coating diameter using polyester elastomer. Further, the delay variation amount in the LN modulator provided with a PMF is larger that of the PMF as a single piece, and a delay amount in the LN modulator itself also has the temperature dependence. As the cause thereof, the thermal expansion of a portion fixing a waveguide substrate and the optical fiber, the thermal expansion of the waveguide length and the like, can be considered.
The next table 1 shows thermal expansion coefficients of an optical fiber core and a coating material, and optical differential delays at the time of temperature variation, which are converted from values of the thermal expansion coefficients.
TABLE 1optical differential delaythermal expansionconverted value (ps)coefficient(length: 0.5 m,material(×10−6/° c.)temperature difference: 80° c.)silica (fiber core)0.50.1UV curing resin50~500 10~100polyester60~95 12~19elastomerpolyamide80~15016~30
According to the above table 1, the thermal expansion coefficient of silica (optical fiber core) is significantly small, and a value of optical differential delay occurred depending on the temperature variation of the optical fiber core itself is significantly smaller in comparison with the measurement result shown in FIG. 29. On the other hand, the thermal expansion coefficient of the fiber coating material is large, and a value of optical differential delay occurred depending on the temperature variation of the coating material is larger in comparison with the measurement result shown in FIG. 29. Thus, it can be considered that the thermally expanded fiber coating extends the fiber core, so that as a resultant, an intermediate differential delay is given to the optical signal.
FIG. 30 and FIG. 31 show one example of measurement results of phase shift tolerance between drive signals (data/clock) and received signal waveforms, in the respective optical modulator on a transmission side for the system of CSRZ-DPSK modulation of 43 Gbit/s. Further, FIG. 32 and FIG. 33 show one example of measurement results of phase shift tolerance between drive signals (data/clock) and received signal waveforms, in the respective optical modulators on a reception side, for the system of RZ-DQPSK modulation of 43 Gbit/s.
In the case where the permissible Q penalty is 0.3 dB, the phase shift tolerance is ±2 ps in the CSRZ-DPSK modulation format (FIG. 30), while being ±5 ps in the RZ-DQPSK modulation format (FIG. 32), and accordingly, has a strict value in either of the systems. Therefore, it is understood that if the delay deviation depending on the temperature change of the PMF as shown in FIG. 28 occurs, the delay deviation depending on the temperature change has a value which cannot be neglected, in comparison with the above tolerance, so that the signal deterioration occurs (FIG. 31 and FIG. 33).
Further, not only the temperature variation of delay amount in the PMFs which connect between the plurality of optical modulators but also the temperature variation of delay amount in an electronic circuit or an electric signal transmission path (for example, an electric coaxial cable or the like) is a cause of signal deterioration.
Namely, in an active device and a passive device used for the electronic circuit, characteristics thereof are changed depending on the temperature, and a bias current and a bias voltage in an amplifier using such devices are varied, so that a delay time is changed depending on the temperature. FIG. 34 shows one example of measuring the temperature dependence of the delay amount variation occurring in an electronic integrated circuit (including a clock distribution IC, a logic IC and the like). This example shows a characteristic in that the delay time is increased with the temperature rise.
Further, the delay time in a route through which the electric signal is transmitted is calculated based on a ratio between a square root of effective relative dielectric constant of the route and a velocity of light, and the route length. However, the route length and the effective relative dielectric constant are changed depending on the temperature, and therefore, the delay time of the route is also changed depending on the temperature. To be specific, the effective relative dielectric constant of the route is decided based on a dielectric constant and a shape of dielectrics to be used for the route, and as generally known, the dielectric constant is changed relative to the temperature, and also, the shape itself of the route is also changed depending on the temperature. Therefore, the delay time in the route through which the electric signal is transmitted has the temperature dependence. FIG. 35 shows one example in which the temperature variation of delay time in a high frequency coaxial cable is measured.
Moreover, not only the temperature dependence of delay amount in the PMF, the electronic circuit, or the electric signal transmission path, but also the length of each of the PMFs connecting the plurality of optical modulators is changed due to a mounting arrangement or the splice processing of each optical modulator. Therefore, there is a problem in that the delay deviation occurs between the respective drive signals. Such a problem is hard to be solved by the conventional technology for directly monitoring the delay deviation between the drive signals to be applied on each optical modulator to perform the feedback control.
In addition, in the conventional technology, since an expensive high-speed device is necessary for monitoring the delay deviation, and therefore, there is a drawback of high cost of the optical transmission apparatus.